This invention relates to optimally switching chillers into and out of a refrigeration system in a building, and, more particularly, controlling the switching of these chillers as a function of at least the design refrigerant head and the actual refrigerant head of the chillers. To increase efficiency, the chillers are switched as a function of, not only design refrigerant head and actual refrigerant head, but also minimum refrigerant head, the capacity of the refrigerating system at the design refrigerant head and the capacity of the refrigerating system at minimum refrigerant head.
In order to meet the air-conditioning needs of large commercial buildings, refrigeration systems are provided having a plurality of chillers. It is apparent that in such systems comprising more than one chillers, it becomes necessary to decide how many chillers to use based upon present conditions and when additional chillers should be connected to or disconnected from the refrigeration system. Prior art systems were rudimentary in their approach to switching between plural chillers to satisfy the refrigeration needs of the building. If they were not simply switched into and out of the refrigeration system manually according to need, simple systems based upon actual building loads were used. If such a simple system comprised three chillers, for example, and if the actual building load was less than 33% one chiller was operated in the refrigeration system; but if the actual building load exceeded 33% of the total design capacity of the three chillers, two chillers were operated and all three chillers were operated if the actual building load exceeded 66% of the total design chiller capacity. If the actual building load dropped below the 66% capacity, the third chiller was disconnected and, if the actual building load dropped below the 33% capacity the second chiller was disconnected. These types of chiller controls do not optimize chiller performance, however, and do not conserve on the electrical energy needed to drive the chillers for providing refrigeration because the arbitrary 33% and 66% switchover points do not reflect optimized performance and may result in chillers being switched on or off when extra capacity is available from the machines operating just prior to switching. The system according to the present invention provides a more sophisticated system for determining the number of chillers which should be operated at any given time. The decision for switching is based upon actual conditions and the most efficient switchover points rather than switching chillers based upon a fixed percentage of total design capacity.